Why? On alliancerom, I have had to do so much work to get it close to the CM experience I had on the S2, S3, and S4. Nothing can come close For example, I need a fast camera...and although the stock rom camera is fast, in the custom roms I tried, the camera in my experience was slow (tried 4 different cameras).
The S4 is actually quite respectable compared to the S5, especially if running CM or other rom.
Don't get me wrong about TouchWiz, on my Note 10.1 2014, I updated to latest KitKat update, which is Touchwiz, however in phone implementation, there just isn't enough customization. Yes I tried GravityBox, Xposed, and the lot, but it just wasn't the same.
Maybe I'll be back for the S5 Prime if it comes out...but my next might end up being the Asus PadFone X...we will see!